Process for treating wood

ABSTRACT

Lumber and other wood products are impregnated with one or more additives such as preservatives, fire retardants, dyes, and the like, using a halogenated hydrocarbon solvent as the sole heat transfer medium and carrier by heating the wood with hot solvent vapor, soaking the hot wood in a pressurized solvent solution of the additive, and releasing the pressure after removing the solvent solution from contact with the wood, thereby causing most of the solvent in the wood to flash off. The process is preferably run in essentially continuous manner by operating three or more interconnected treating chambers simultaneously in offset cycle sequence using a common source of boiling treating solution under moderate superatmospheric pressure.

United States Patent Dunn, Jr. 1 Aug. 19, 1975 1 1 PROCESS FOR TREATINGWOOD [75] Inventor: James L. Dunn, Jr. Lake Jackson Primarybmnlmerw1ll1am R. Trenor Attorney, Agent, or Fzrm--G1wynn R. Baker [73]Assignee: The Dow Chemical Company, 57 ABSTRACT Midland, Mich. 1 Lumberand other wood products are impregnated 122] Flled: 1972 with one ormore additives such as preservatives, fire [21 APPL Nu: 297.464retardants, dyes, and the like, using a halogenated hydrocarbon solventas the sole heat transfer medium and carrier by heating the wood withhot solvent va- [52] U.S. C1. 427/317; 8/65; 21/7; por Soaking the hotwood i a pressurized Solvent 34/95; 34/13-8; 34/16-5 lution of theadditive, and releasing the pressure after 51 11 C1 B27k 3/08; B271 3/40removing the Solvent solution from Contact with the [58] Fleld of Search21/7; 1 17/59, 147, 149, wood thereby Causing most of the sowem in thewood 1 8/65; 34/95 to flash off. The process is preferably run inessentially continuous manner by operating three or more inter- 1561References C'ted connected treating chambers simultaneously in offsetUNITED STATES PATENTS cycle sequence using a common source of boiling2,860,070 11/1958 McDonald 117/147 treating Solution under moderatesuperatmospheric 2,931,737 4/1960 Thornton 117/147 pressure. 3,200,0038/1965 Bescher 117/59 1 3,685,959 11 1972 Dunn 21 7 5 Clams l DrawmgF'gure I f WO/E/Ou/ -g s Wmer in 2 "5102. 1'

7'0 619/: pressure 24 condenser 24 1 24 V V 3 b 24b 0 26a 2 255gOVEf/Ieoo vapor C rans er V r\ 2 v urye V 1 Sur e drum dru /n 1 5 25Cy/ina er 2 Cy/fnaer 2 (y/b70616 1/0 or 1 a 25) V frr nsper 25 *M"! 25a2 250 5 J Q Z7 4/ u/a Low ress. 9 vraZzS/sr cono nser 27c Reba/Yer gl'n/ef 22b V 27 0 22a Q 21 Reboi/er calf/e); 315 216 21a 1 7/175 17} V5/ eam #1817 18a //'7 Low ressure 1,

' COHZZ'QS {e Corifienser 18 n O 0a Pen/a Mak up 186 V E/ven/ 19b 20 I Wfresh fso/vep/ 20a f ue/7 am/x PROCESS FOR TREATING WOOD BACKGROUND orTHE INVENTION This invention'relates to' a, process wherebyi wo od isimpregnated with modifying additives suchas preservatives, fireretardants, dyes, and the like. It relates particularly to anessentially continuous process utilizing methylene chloride or a similarhalogenated solvent as the carrier and heat transfer agent forimpregnating wood with pentachlorophenol or other preservatives, dyes,fire retardants, and the like.' 7 v A number of processes are known inwhich-halogenated hydrocarbon solvents such as perchloroethylene,trichloroethylene, and methylene chloride have been used as theprincipal carriers for wood'treating agents. Representative of theseprocesses are those described by Hudson, US. Pat. No. 2,650,885, TannerCanadian Pat. No. 519,730; McDonald, US. Pat. No. 2,860,070; and Dunn,Canadian Pat. No. 863,885. All of these processes, by thenature of theirsteps and the time required to complete a single cycle of operation,i.e., treatment of a single charge of Wood, are either unsuited to largescale general use ,or applicable for commercial treatment of only a fewkinds of wood. For example, Hudson and McDonald use the relatively highboiling perchloroethylene and trichloroethylene which are not readilyremovable from the interiorof the treated wood even when vacuumtreatment isapplied. Tanner describes a solvent impregnation of woodapparently carried out at ambient temperature with solvent removalcarried out by any of various means with no mention of solvent recovery,evidently a slow process which would be wasteful of solvent. Dunndisclosesa commercially operable process using methylene chloride as thetreating medium, but one which would apparently have inefficientutilization of steam and cooling water requirements. I

The present process is a modification of the latter procedure wherebyrapid and essentially continuous treatment of wood is accomplished withhighly efficient use of both energy and treating materials. lt isapplicable to both the more permeable woods such as southern yellow pineand ponderosa pine and the denser hard woods. I

SUMMARY OF INVENTION It has now been found that the above desirableobjectives are obtained by a process which comprises contacting wood atabout 50C. to about 125C. with a chlorinated aliphatic hydrocarbonsolvent vapor under superatmospheric pressure for a time sufficient toheat the wood to approximately the temperature of the solvent vapor,immersing the heated wood in a boiling solution of at least one woodtreating agent in said chlorinated solvent at said temperature range andunder superatmospheric pressure for a time sufficient to causesubstantial impregnation of the wood by the solution,

2 pressurized boiling treating solution, using pressurized solvent vaporfrom a vessel in the wood soaking part of the treating cycle to preheatnew wood in another vessel, and having common means for condensingsolvent vaporvand recycling the condensate in the process, and a commonsolution make-up tank.

'In such a mode of operation, energy requirements per treating vesselare minimized and a substantially continuous process is obtained whereinhand labor is reduced essentially to loading new wood into the treatingvessels and unloading the treated wood.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a diagrammaticrepresentation of a preferred wood treating apparatus of the inventionhaving three treating chambers in the form of horizontally disposedcylindrical presure vessels, each having one closed end and an openendfitted with a door for loading and unloading wood, the doorsbeingcapable of being closed and sealed to withstand moderatesuperatmosphericpressures. The three cylinders have a common source 'ofhot treating solution (the reboiler), a common solution make-up tank,common condensing means for solvent vapor at about atmospheric pressureand another such means for condensing solvent vapor at superatmosphericpressure, and appropriate circulating pumps for treating solution, allofthe above with suitable interconnecting lines and valves,gtemperatureand pressure regulators, ventsand drains as required.

DETAILED DESCRIPTION Thefollowing description of apparatus suitable forcarryingout the present invention has particular reference to thedrawing. Three pressure cylinders '10, I1 and 12 are provided, each witha door or end, not shown, through which the wood to be treated can beloaded or unloaded into the cylinder, and which door will sealthe-cylinder in a manner to prevent escape of liquid or vapors from thecylinder under superatmospheric pressure. Each cylinder l0, l1 and 12 isprovided-with a surge drum l3, l4 and 15, respectively. A highpressure-water cooled condenser 16 is connected through piping 24 toeach surge drum 13, 14 and 15. A temperature responsive control valve16A maintains the pressure within the condenser 16 through tempera-vture sensing element 168. The condensate from condenser 16 passes topump 16D through line 16C to reboiler 17. Reboiler 17 is connectedthrough piping 21 to the three cylinders 10, 11 and 12. Each cylinder10, 11 and 12 is connected to a low pressure condenser 18, having atemperature responsive control 18A associated with temperature sensingelement 18B. The condensate from the low pressure condenser 18 isdirected to fresh solvent and penta mix storage tank 19. Penta make-upand fresh solvent make-up are introduced into tank 19 through lines 19Aand 193, respectively. Pump 20 withdraws solvent from tank 19 on demandand delivers it to reboiler 17. Various other piping is provided in aninterconnecting manner to transfer fluids from one cylinder to another.Valves are provided to control the flow of fluids. These valves arepiping are fully identified and their operative cycles are set forth inthe following description and tables.

The present invention is illustrated hereafter with regard to treatingyellow pine dimension lumber. Other woods require different treatingcycles. However, it can generally be determined that the averagetreating cylinder, for example, requires about 30 minutes to load and 3an equal period to unload; that a normal cylinder takes about 75 minutesto purge of air and heat the lumber to the treating temperature; thatthe equipment (pumps, piping, etc.) limits the rate of filling thecylinders with TABLE I Step I, Time Units 1-2 Cylinder 2 Cylinder 3Cylinder l liquid and the build-up of pressure such that filling rewmdfi i' has and cylinder door is pressurized boiling drained from soakedqulres about mml 1te5' It known'that y q p closed solution admitted woodinto Cylinder 3 lumber bundles require about 105 minutes to impregfromCylmdcr 3 prcssurqls reduced I to atmosphenc through natethe bundle.Draining, solvent recovery and letlow pressure condenser down toatmosphere are about equal in total time reto flash ofl'solvent in thequirement to filling, soaking and purging. An equation to enable theoperator to adjust his cycles to a multiple Step 2, Time Units 3-7 li dOperation ld t k th f f; Purge air from Wood remains Solvent is flashedoff as cylinder with hot soaking above v solvent vapor from 2L P 2FD .SR/no. cyl.- total cycle time Cylinder 2 where Step 3, Time Units 8-10 Ltime IO load 01 unload Fill with pressurized Drain solution to l Admitair, open door P time to purge and heat i vapors lzooiling solution fromand remove wood FD time to fill or drain s ti t k Step 4, Time Unitsll-l7 Wood soaks Reduce to Load in new wood, R time (0 recover solventatmospheric purge air and heat wood Substituting the times in minutes weset forth above, we prtlzssurehflashhoff twith hot solvent vapor so ventt roug low mm 1 find the time for a single cycle in a single cylinderpressure condenser equa s Step 5 Time Units 18-20 2( 2(45) 450 minutes(or 7" 25 Drain solution to 3 Admit air. open to Admit solution from I,

cycle for one cylinder. remove wood wood soaks K th' d d idi t u e three0 linders we stcp 'Timc 2] f g an ng O S Flash ofi solvent Load in woodWood soaks can divide the cycle into three equal periods of from woodthrough minutes (2 hour 30 minutes) and thus know the second d cylindershould be ready to load and start a cycle 2' 3O reduce to 30'" after thefirst cylinder was opened for loading and mmosphcflc Pressure h Step 7,Time Units 22-26 the third cylinder should be ready to load 5 followingFlushing off as in Purge air and hem 7 wood Soaking the first cylinderzero time. This permits utilization of Time Un t 21 wood Withdholl thecylinders to exchange vapors treating liquid, etc., 35 3 conservingenergy while accomplishing adequate treat- Step 8. Time Units 27-28 ingReduce to Heat wood with Wood soaking then atmospheric vapor from 3,admit drain solution to 2 An offset cycle sequence is described indetail in Tapressure admit hm 80min f 3 bles l and ll for the threecylinder apparatus of the U d )d svtspojfTingc Units 29-30l d d 7drawing. The individual operating steps are allotted 5 0 certain numbersof time units as an indication of the ap- 40 roximate relative len thsof the ste s. This would var p I p y Table II lists the valving sequenceduring the treating depending upon the dimensions of the wood pieces andn v procedure of Table I. For each valve, an O marked the kind of woodparticularly. For a representative under a particular time unitindicates that the valve is wood treating procedure, each time unitmight be 15 minutes for example open. A valve not so marked is closed.

1 TABLE ll 'TimeUnit l 2 3 4 5 6 7 s 9 10 ll l2 l3 l4 15 Valve Not A 0 00 0 0 21 B 0 0 0 0 0 o 0 0 0 0 C A 0 0 0 0 0 22 B 0 0 0 0 0 0 0 0 0 0 CA 0 0 0 23 B 0 0 0 C A 0 0 0 0 0 0 0 0 0 0 24 B 0 0 C 0 0 0 A 0 0 0 B 00 0 0 0 25 C D 0 0 0 0 0 E F 0 0 0 A 26 B C 0 0 A 27 B o 0 0 0 0 C u 0 00 0 o 0 0 TABLE ll-continued f 1 Time Unit Vulvc No. v i

C v 0 0 i1 0 0 0 0 .0 A 0 0 u 0 1' 22 B Y 11 C 0 0.. o .0 0 0 (1, 0 Ao 1) 0 23 B 0 0 0 C .0 0. 0 0 0 -0 A v 24 B '0 (1 0 0 0 0 0 0 A 0 0 B 25C 0 0. 0 0 0 26, k B 0 0 g C i 4 v 1 A I o 0 0 0 0 0 0 0 0 0 In atreating procedure as outlined in Tables I and II carbon tetrachloride;the :polychloroethanes, as using aboiling methylene chloride solution ofpentaethylene dichloride, methylchlorof0rm ;.;and-l,l',2=trichlorophenolas the treating solution and operating ata' chloroethane; and thepolychloroethylenessuch as 1, 2 solvent vaportemperatu're' of about 78C.and a gauge 'dichloroethylene, trichloro ethylene, andperchlorqethprssu of u 4 q iniin th WOO'd i p g ylene. Other chlorinated aliphaticsolventsjwh-ieh can n t p. typical operating pressur s in l/ q: n. 30beused are propylene dichlorideand compounds-as deg ug in yl n r i theoiler, and in the high scribed above where some of-the chlorineatomshavepressure condenser are about as followsin the various been'replaced byfluorine-{for example, l, 2 d ichloro"- steps. l1,1,2,2-tetrafluoroethane, dichlorofluoromethane{and TABL "Itrifluoromethane. Since an essential feature Qfthispicess is the use ofsolvent vapor and treating solu t1on at I *Stcp' Prcssure,psig r t Y. II No; Cylinder] A Rcboilcr H.P.Condenscr temperaturesSlgnlficantlyv'abqve P p p point of the solvent, a solvent-having arelat1yely..-,low l .1 64 boiling point-is'preferred,.for example, inthe rangeof- 2 initially (l, 62 30 I Q then to 30 about 35 -80C.Methylene chloride is particularly-pre- 3 30-64 ferredi 1 4 t 30 f: 1 6462 6 2 Pentachlorophenol or a mixture of pentachlorophe- 6 5041 64 64nol with a minor proportion of tetrachlorophenol is the Z, 23 2 28 3preferred wood preservative used in the process. Pref- 9 0 63 64 erably,the treating solution contains about 2-10 percent by weight ofpentachlorophenol. Other wood preserving chernicals can be used, eitheralone or in com- The preliminary heating step wherein the wood isbination with pentachlorophenol in the treating solucontacted with hotpressurized solvent vapor is preferation. Additionally, the treatingsolution may contain bly carried out at about 100C. and a pressurecomponents such as fire retarding chemicals, dyes, or from about 1.] toabout 5 atmospheres. This step re- '50 other such additives to modifythe properties of the quires from about 0.5 hour to about 4 hoursdepending treated wood. upon the temperature employed, the kind of wood,the The process is most advantageously used for impregdimensions of thepieces, and the amount of moisture nating seasoned wood withpreservatives, fire retardant in the wood. chemicals, dyes, or otherwood treating compositions The same preferred temperature range appliesto the or mixtures thereof. However, it can also be applied to soakingstep in which the hot wood is immersed in sugreen or partially seasonedwood in which case it beperheated treating solution. Somewhat higherprescomes a combined seasoning and impregnating prosures are preferredfor this step, for example, up to cess. Somewhat longer heating andsoaking steps may about l5 atmospheres. Soaking time varies accordingthen be necessary. to the degree of penetration wanted, the dimensionsof EXAMPLE the pieces being treated, and the moisture content anddensity of the wood. Soaking times as long as 10 hours The followingexample describes the procedure used can be employed, but preferably thesoaking time is and the results obtained from a single-cycle ofoperaabout 0.5-3 hours. tion adaptable to multiple chamber operationwhere Chlorinated solvents suitable for use in this process are thosehaving boiling points between about 35C. and about C. and these includethe polychloromethanes such as methylene chloride, chloroform, and

the individual cycles are staggered and coordinated such that thetreating solution filling time for one chamber coincides with thesolution removal time of another.

The treating chamber was a horizontally disposed steel cylinder about 12feet long and 2 feet in diameter with one end permanently closed and asealable door at the other end. It contained a steam coil along thebottom, a tram for moving lumber into and out of the cylinder, and itwas equipped with appropriate piping connections for admission andremoval of liquid and vapor, including connections to a water-cooledcondenser and liquid storage. The whole system was adapted to operationunder moderate superatmospheric pressure.

Ten seasoned yellow pine two-by-fours eight feet long were fastened inplace on the tram within the cylinder, the cylinder door was closed, andenough pentachlorophenol in methylene chloride was introduced to coverthe steam coil. Steam was passed into the coil to heat the solution toboiling and the air in the cylinder was vented as it was displaced bymethylene chloride vapor. After the air had been vented from the system,heating was continued so as to maintain the treating cylinder at about78C. and 30 p.s.i.g. After 3.75 hours under these conditions when thelumber was heated through, additional treating solution was pumped intothe cylinder to cover the wood. Steam pressure in the coil was thenincreased to obtain a final temperature "of 91C. and a pressure of 64p.s.i.g. within the-cylinder. Thewood was subjected to the actionof'the' solution under these conditions for 45 minutes. The steam wasthen shut off and the solution was drained from the cylinder into astorage tank. The cylinder-was then vented through a water-cooledconderiser,-'whereupon' most of the solvent in the hot lumber vapo rizedto be condensed in the condenser and collected;

"moved the residual solvent. .The dried two-by-fours contained about onepound of pentachlorophenol per cubic foot and showed good penetration.The treatment'had also reduced the water content of the lumber 8 from15.2% to 13.4%. The treated wood showed no checks or cracks and therewas little or no discoloration.

I claim:

1. A process for treating wood which consists essentially of the steps:

a. contacting wood at about 50C. to about C. with a vaporizedchlorinated aliphatic solvent under superatmospheric pressure for a timesufficient to preheat the wood to approximately the temperature of thevaporized solvent,

b. immersing the heated wood in a solution of at least one wood treatingagent in said chlorinated solvent at about said temperature and undersuperatmospheric pressure for about 01-10 hours,

c. removing the excess solution from contact with the therebyimpregnated wood, and

d. reducing the pressure on the impregnated wood at least to atmosphericpressure, thereby causing vaporization of at least a substantial part ofthe solvent contained in the wood.

2. The process of claim 1 wherein the solvent vaporized from the treatedwood is condensed and recycled to the process.

3. The process of claim 2 wherein the solvent is methylene chloride.

4. The process of claim 3 wherein the wood treating agent ispentachlorophenol. i

5. The process of claim 1 wherein the heated wood is immersed in aboiling wood treating solution under superatmospheric pressure and thehot solvent vapors from the boiling solution are used at least in part,also under superatmospheric pressure, to contact and preheat a secondbatch of wood in the treating process, said second batch of wood afterthus having been preheated then being immersed for treating undersuperatmospheric pressure in the boiling solution removed from the firsttreated wood, thereby obtaining an essentially continuous wood treatingprocess.

1. A PROCESS FOR TREATING WOOD WHICH CONSISTS ESSENTIALLY OF THE STEPS:A. CONTRACTING WOOD AT ABOUT 50*C. TO ABOUT 125*C, WITH A VAPORIZEDCHLORINATED ALIPHATIC SOLVENT UNDER SUPERATMOSPHERIC PRESSURE FOR A TIMESUFFICIENT TO PREHEAT THE WOOD TO APPROXIMATELY THE TEMPERATURE OF THEVAPORIZED SOLVENT. B. IMMERSING THE HEATED WOOD IN A SOLUTION OF ATLEAST ONE WOOD TREATING AGENT IN SAID CHLORINATED SOLVENT AT ABOUT SAIDTEMPERATURE AND UNDER SUPERATMOSPHERIC PRESSURE FOR ABOUT 0.1-10 HOURS,C. REMOVING THE EXCESS SOLUTION FROM CONTACT WITH THE THEREBYIMPREGNATED WOOD, AND D. REDUCING THE PRESSURE ON THE IMPREGNATED WOODAT LEAST TO ATMOSPHERIC PRESSURE, THEREBY CAUSING VAPORIZATION OF ATLEAST A SUBSTANTIAL PART OF THE SOLVENT CONTAINED IN THE WOOD.
 2. Theprocess of claim 1 wherein the solvent vaporized from the treated woodis condensed and recycled to the process.
 3. The process of claim 2wherein the solvent is methylene chloride.
 4. The process of claim 3wherein the wood treating agent is pentachlorophenol.
 5. The process ofclaim 1 wherein the heated wood is immersed in a boiling wood treatingsolution under superatmospheric pressure and the hot solvent vapors fromthe boiling solution are used at least in part, also undersuperatmospheric pressure, to contact and preheat a second batch of woodin the treating process, said second batch of wood after thus havingbeen preheated then being immersed for treating under superatmosphericpressure in the boiling solution removed from the first treated wood,thereby obtaining an essentially continuous wood treating process.